1. Field of the Invention
The subject invention relates to a method and apparatus for use in a press assembly and, more particularly, to a cushion assembly which provides a yieldable force during operation of the press assembly from an open condition to a closed condition.
2. Description of the Invention Background
A variety of different products and components are manufactured utilizing apparatuses called "dies". A die can comprise a complex and expensive device that punches holes, cuts, bends, forms, etc. raw material (e.g., sheet metal and the like) that is placed within the die. For example, automobile fenders, side panels, etc. are typically formed from sheet steel that is placed within a die.
A die is typically operated by a mechanical pressing mechanism that can generate large amounts of force for pressing the die components together when the raw material is placed therein. A typical mechanical press can generate tons of pressing force depending upon its design. Most mechanical presses employ a large rotating flywheel arrangement and use a crankshaft or eccentric shaft to convert the rotary motion of the flywheel to a straight line pressing motion which is applied to a slide that contacts a portion of the die. The geometry of this combination of parts results in a changing mechanical advantage between the drive and the slide. For example, the mechanical advantage of the crank arm and connection assembly will vary from one, at a point near midstroke, to infinity at the bottom of the stroke.
The impact forces and associated shock loads created during the pressing process can result in undesirable wear and damage to various die and press components. Thus, to reduce die wear and damage, which can lead to costly down time and maintenance expenses, cushion assemblies have been employed to support the die on the machine and absorb a portion of the shock forces created thereby. U.S. Pat. No. 4,792,128 and U.S. Pat. No. 4,838,527 to Holley disclose various types of cushion assemblies.
A known cushion assembly, that is, a gas spring is also depicted in FIG. 1. As can be seen in FIG. 1, the cushion assembly 10 comprises a body 12 that slidably supports a piston 16 therein. The piston 16 is attached to a piston rod 18 that is oriented in the mechanical press to engage the bottom of the die or a movable table (known as a pin plate) that supports the die. The body 12 is hollow and is capped on one end by a cap 14. The cap 14 and the bottom of the piston 16 cooperate to define a gas chamber 20. The gas chamber 20 is charged with a compressible gas, such as nitrogen, through a conventional pressure valve 22 located in the cap 14. The piston rod 18 is slidably supported within the body 12 by a rigid rod support member 24 that is typically fabricated out of metal such as bronze. The upper end 13 of the body 12 is sealed with a retainer cap 26 that is affixed in position with a conventional retaining ring 28. As illustrated by arrow "A" in FIG. 1, the gas pressure within the chamber 20 serves to push the piston rod 18 out of the body 12 until the piston 16 contacts the rigid retainer 24. Such construction results in the preloading of the piston rod 18 such that the development of a contact force (i.e., the force required to initiate movement of the spring from the fully extended position) on the end of the piston rod 18 is required to compress the gas spring 10. Conventional cushion assembly designs deliver almost full force at contact and have relatively little force increase as the cushion assembly is compressed. This results in the application of high forces instantaneously at contact with the rod 18. This instantaneous force loading is transferred to the other components of the press and results in undesirable shock loading of the press and die.
FIG. 2 is a press tonnage curve of a typical mechanical press wherein known cushion assemblies of the type described above are employed. The vertical axis represents the amount of force (tonnage) generated by the press and the horizontal axis represents the distance that the press slide is away from its bottom limit of travel. As can be seen from that graph, the press load capacity increases along an arcuate slope to a certain point as the distance between the press slide and its bottom position increases. The tonnage signature is the actual application of forces by the press during operation and takes into account a variety of process variables such as speed, overloading, etc. The preload of the cushion assemblies can result in the undesirable shock overload depicted in FIG. 2 wherein the actual load exceeds the press load capacity. Such shock loading can result in die and press wear and failure, excessive noise, and undesirable pad bounce.
The preload characteristics of conventional cushion assemblies or gas springs of the type shown in FIG. 1 are a major contributor to the generation of shock loads and noise as well as excessive pad bound on the return stroke. In an effort to reduce the magnitude of initial contact forces applied to the piston rod, cushion assemblies that employ a floating piston (e.g., a piston that is not attached to the piston rod) and a dual gas chamber arrangement have been developed. The additional gas chamber purportedly serves to balance the net force on the piston rod at its fully extended position.
Swedish Pat. No. 9401119-4 discloses a cushion assembly that employs a floating piston and a dual gas chamber arrangement. This reference also teaches that a damping body may also be employed to "further damp any residual noise." Such a cushion assembly requires additional sealing elements to be employed and produces undesirable dynamic effects due to inertia of the floating piston, and static friction of the seals.
FIG. 3 is a graphical comparison of the theoretical force curves of a conventional cushion assembly or gas spring of the type depicted in FIG. 1 and the above-mentioned spring that employs a floating piston. As can be seen from that Figure, the floating piston represents a modest improvement over the standard gas spring in that its initial curve (upon contact) is slightly sloped when compared to the essentially vertical curve of the conventional gas spring.
Another approach that has been employed to reduce gas spring contact force involved the use of cushion assemblies, that is, gas springs, that have stepped pistons. Such a spring is disclosed in U.S. Pat. No. 5,129,635 to Holley. FIG. 4 is a graphical comparison of the force curves of a conventional gas spring of the type depicted in FIG. 1 and a conventional gas spring that employs a stepped piston. As can be seen in that Figure, however, the undesirable instantaneous step function in the spring load is not eliminated when utilizing a spring with such a piston arrangement.
Thus, there is a need for a cushion assembly that has an improved shock loading characteristics during operation of a press assembly.
There is a further need for a press cushion assembly that can be constructed to provide a desired contact force while reducing undesirable shock during initial operation of the cushion assembly.
There is still another need for a spring arrangement with the above-mentioned characteristics that is relative easy to manufacture and service.